Hydrocarbon exploration typically involves various geophysical methods to detect the presence of hydrocarbons in the natural void space of the rock (measured as “porosity”) or to map structural features in a formation of interest which are capable of trapping hydrocarbons.
To be mapped geophysically, the formation containing the hydrocarbon must possess a physical property contrast that the geophysical method responds to. For example, seismic waves reflect off interfaces between rock types that have different seismic impedance (the product of velocity and density). The velocities of seismic waves traveling in a medium depend primarily on the elastic constant and density of the medium (e.g., rocks). The density of the medium in turn depends on the porosity and fluid content. The seismic wave velocity is sensitive to porosity, but not sensitive to the type of fluid in the pores, although it is sensitive to the presence of gas.
The electrical conductivity (σ), or its inverse, resistivity (ρ), is a physical property that can be measured with electrical or electromagnetic (EM) methods. The resistivity of a rock depends strongly on the resistivity of the pore fluid and even more strongly on the porosity of the rock. Typical brine in sedimentary rock is highly conductive. The presence of brine in bulk rock renders the rock conductive. Hydrocarbons are electrically non-conductive. Consequently, bulk resistivity of a rock is reduced when hydrocarbons are present. In general, different rocks in a given sedimentary section have different porosities, so even in the absence of hydrocarbons, information about the sedimentary section can be determined. The combination of seismic and resistivity data is useful in assessing hydrocarbon content.
Resistivity is typically measured with a direct current (DC) source that injects current into the ground or with low frequency time varying fields. The former currents satisfy Laplace's equation and the latter satisfy the diffusion equation. The electromagnetic (EM) methods described herein may use DC and/or diffusion fields.
Diffusion of time-varying electric and magnetic fields in a conductor is basically an induction phenomenon. In EM measurements, a current is made to flow in a formation, and the voltage drops produced by the current is then measured. Alternatively, one may measure the magnetic fields produced by the induced current. Currents can be made to flow in the formations by injection using contacting electrodes. A current injection source produces a DC response. Alternatively, currents may be made to flow in the formations by using an inductive source, i.e., by creating a time-varying magnetic field that induces an electromotive force (emf) or voltage in a conductor (e.g., an earth formation) according to Faraday's Law. The induced emf or voltage in turn drives a secondary current (eddy current or Faraday current) whose magnitude depends on the conductivity of the conductor (e.g., the earth formation). Thus, by measuring the magnitude of the induced current or the secondary magnetic fields arising from these, it is possible to infer the conductivity of the earth formation. Other important sources for inducing current flows in an earth formation are natural electromagnetic fields. The present invention relates to methods for using these sources/fields to determine resistivities in a given geologic section in the formations.
In order to fully explain the present invention, description of related prior art techniques is required.